Check valves of the type known as “duck-bill” check valves have been used successfully for a very long time in the control of the flow of fluid where it is desired to pass fluid through a conduit in one direction and prevent flow in an opposite direction. In particular, duck-bill check valves have been coupled to effluent pipes to allow the flow of effluent out of such a pipe, while preventing reverse flow, or backflow, into the pipe as a result of a build-up of pressure downstream of the valve. These valves employ a resiliently flexible tubular valve body having a bill-like section terminating at the valve outlet. The outlet is biased into a closed configuration by the resilient nature of the valve body, and opens in response to fluid pressure within the pipe to discharge flow through the outlet. The outlet is closed upon build up of back pressure downstream of the valve.
Duck-bill check valves, while relatively simple in design and construction, exhibit some drawbacks, especially when provided in the larger sizes employed in connection with large effluent pipes, such as those found in storm drain systems. A common problem is inversion, that is, upon a build-up of back pressure at the outlet of the valve, the bill will tend to collapse and invert, thus opening the valve to backflow. Attempts to stiffen or reinforce the bill against inversion have resulted in the requirement for greater pressure upstream of the valve in order to open the valve for the discharge of effluent, with a concomitant build-up of effluent in the system upstream of the valve. In addition, the accumulation of larger volumes of effluent within the valve itself leads to deleterious sagging of the valve due to the weight of the effluent contained within the valve.